Formation of Cyclopropanes via Activation of (γ-Methoxy)alkyl Gold(I) Complexes with Lewis Acids

Abstract

Treatment of the gold 3-methoxy-3-phenylpropyl complex (P)­AuCH2CH2CH­(OMe)­Ph [P = P­(t-Bu)2 o-biphenyl] with AlCl3 at −78 °C led to the immediate (≤5 min) formation of a 4:1 mixture of phenylcyclopropane and (1-methoxypropyl)­benzene in 86 ± 5% combined yield. Lewis acid activation of the stereochemically pure isotopomer erythro-(P)­AuCH2CHDCH­(OMe)­Ph led to the formation of cis-2-deuterio-1-phenylcyclopropane in 84 ± 5% yield as a single stereoisomer, which established that cyclopropanation occurred with inversion of the γ-stereocenter. Similarly, ionization of the stereochemically pure cyclohexyl gold complex cis-(P)­AuCHCH2CH­(OMe)­CH2CH2 CH2 at −78 °C formed bicyclo[3.1.0]­hexane in 82% ± 5% yield, which validated a low energy pathway for cyclopropanation involving inversion of the α-stereocenter. Taken together, these observations are consistent with a mechanism for cyclopropane formation involving backside displacement of both the Cγ leaving group and the Cα (L)­Au+ fragment via a W-shaped transition state.

Conformationally Fixed Chiral Bisphosphine Ligands by Steric Modulators on the Ligand Backbone: Selective Synthesis of Strained 1,2-Disubstituted Chiral cis-Cyclopropanes

“…This bond formation should be possible because the electron occupancy of the opposite lobe of the Cu–C 1 bond would be sufficiently large to interact with the electrophilic moiety. This process could be recognized as a stereoinversive electrophilic substitution (S E 2) at the carbon center (C 1 ) attached to the copper­(I) center . Therefore, the stereogenic alkylcopper­(I) might react with various electrophiles in both stereoretentive and -inversive manners.…”

“…This process could be recognized as a stereoinversive electrophilic substitution (S E 2) at the carbon center (C 1 ) attached to the copper(I) center. 69 Therefore, the stereogenic alkylcopper(I) might react with various electrophiles in both stereoretentive and -inversive manners. This can also explain our previous results involving varied cyclization selectivities.…”

Substituent Effects and the Mechanism of Gold to Alkene Benzylidene Transfer Employing a Gold Sulfonium Benzylide Complex

“…A number of transition state geometries have been proposed and supported by computation for gold to alkene carbene transfer ranging from a perpendicular to an anti -periplanar arrangement of the AuC and CC bonds. − Relevant to this discussion of the transition state geometry for gold to alkene carbene transfer, we have recently reported the formation of cyclopropanes via ionization of (γ-methoxy)­alkyl gold­(I) complexes . Isotopic labeling and related stereochemical experiments established the inversion of the configuration at the carbon atoms α and γ to gold, consistent with a W-shaped transition state involving electrophilic attack of the nascent electrophilic γ-carbon atom on the back lobe of the carbon–gold σ bond (Scheme ), as has similarly been established for cyclopropanation of γ-functionalized alkyl iron complexes. − In a similar manner, the mechanism for gold to alkene benzylidene transfer from I depicted in Scheme also involves electrophilic cleavage of the Au–C bond via the interaction with the nascent electrophilic γ-carbon atom to gold.…”